Tunable oscillatory circuits



Feb. 11, 1941. D. KOFFYBERG 2,231,389

TUNABLE OSCILLATORY CIRCUITS Filed Sept. 27, 1959 2 Sheets-Sheet 1 7011E NETWORK) COMB/NED LOCAL 050. fmvp FIRST DETECTOR I o1 I A g g g: z Il3 1 2 Ill 15 I L 1 3 INVENTOR. 0/5? KOFF YBERG BY ATTORNEY.

Feb. 11, 1 941.

' D. KOFFYBERG TUNABLE OSC'ILLATORY CIRCUITS Filed Sept. 2'7, 1 959 2Sheets-Sheet 2 INVENTORL DIEDERIK KOFFYBERG ATTORNEY.

Patented Feb. 11, 1941 UNITE.

STATES PATN QFFIQE TUNABLE OSCILLATORY CIRCUITS Netherlands ApplicationSeptember 27, 1939, Serial No. 296,726

In Germany May 8, 1939 5 Claims.

The present invention relates to a circuit arrangement for theamplification or production of high frequency oscillations, and moreparticularly to a radio receiving circuit arrangement including atunable oscillatory circuit.

As is well known, the natural frequency of an oscillatory circuitgenerally decreases with increasing temperature, because both thecapacity and the inductivity of the oscillatory circuit and of theelements associated therewith increase with increasing temperature, or,to put it another way, have a positive temperature coeflicient ofelectric resistance. This often annoying temperature dependency of anoscillatory circuit can be reduced or suppressed in a well known manner;for instance, by connecting a fixed condenser, whose capacity has anopposite and consequently as a rule a negative temperature coefficient,in parallel with the oscillatory circuit. If, however, the oscillatorycircuit is tunable, this known circuit for decreasing the naturaltemperature dependency of an oscillatory circuit allows completecompensation of temperature dependency only for a single frequency ofthe tuning range. For higher frequencies there occurs over-compensation,whereas for lower frequencies the natural temperature dependency of theoscillatory circuit is only decreased, but not eliminated. The saidexpedient does not permit the natural temperature dependency of theoscillatory circuit to be reduced throughout the tuning range in a noteven approximately regular manner.

The present invention permits the natural temperature dependency of atunable oscillatory circuit to be reduced in a much more favorablemanner. According to the invention there is connected in parallel with,and in series with, the adjustable reactance for tuning the oscillatorycircuit in each instance an additional reactance, which additionalreactances have the same character as the adjustable reactance and whosetemperature dependency is directed in opposition to that of theadjustable reactance. The size and the temperature dependency of each ofthe additional reactances are preferably so chosen that the naturalfrequency of the oscillatory circuit is independent of the temperaturefor one of the higher frequencies and for one of the lower frequenciesof the tuning range.

In a heterodyne receiving circuit arrangement, including an oscillatorycircuit and a second oscillatory circuit detuned with respect to theformer, each of which oscillatory circuits can be tuned by means of anadjustable condenser and with which for equalization of the oscillatorycircuit of the oscillator an additional condenser is connected inparallel with its tuning condenser, whereas for keeping constant thedifference in natural frequency of both oscillatory circuits a furtheradditional condenser is connected in series with the last mentionedtuning condenser, the two additional condensers receive, according to afurther feature of the invention, a temperature dependency directed inopposition to that of the tuning condenser of the oscillatory circuit ofthe oscillator.

The invention will be more clearly understood by reference to theaccompanying drawings representing, by way of example, some embodimentsthereof. In the drawings, Figs. 1 and 3 represent oscillatory circuitsaccording to the invention; Fig. 2 represents a graph of the temperaturedependency of two oscillatory circuits, Fig. 4 shows a'heterodynereceiving circuit arrangement according to the invention, Figs. 5, 6,and 7 show respectively different simplified embodiments, and Fig. 8shows a system similar to Fig. 4 embodying a simplified modification ofthe invention.

Fig. 1 shows a tunable oscillatory circuit com prising an adjustablecondenser I and a fixed coil 2. As has already been stated the capacityof tuning condenser I and the inductance of coil 2 generally increasewith increasing temperature. For reducing the temperature dependency ofthe oscillatory circuit involved thereby a condenser 3 is connectedinparallel with the tuning condenser, and a condenser 4 is connected inseries with the tuning condenser. Each of the condensers 3 and 4 has acapacity which decreases with increasing temperature or in other words anegative temperature coefficient. In the drawing the negativetemperature coefficient is designated by a negative sign.

Of course, condensers 3 and 4 may be various types of condensers havinga negative temperature coefficient. For this purpose, use may, forinstance, be made either of condensers known per se having a. dielectricwhose dielectric constant decreases with increasing temperature orcondensers whose electrodes are adjustable relatively to one'anotherand, by means of a bi-metal strip or the like, are moved with respect toone another with an increase in temperature so that the capacitydecreases. The capacity of condenser 3 is preferably chosen smaller thanthe minimum capacity of the tuning condenser, and this for limiting aslittle as possible the tuning range which can be attained by means oftuning condenser I. If limitation of the tuning range which can besecured by means of tuning condenser I is not desirable the capacity ofcondenser 4 is to be chosen materially higher than the maximum capacityof the tuning condenser.

The influence of temperature dependency on the additional condenser 3 isgreater as the capacity of tuning condenser I is smaller and willconsequently be greater as the adjusted natural frequency of theoscillatory circuit is higher. When the capacity of condenser 3 is smallwith respect to the maximum capacity of the tuning condenser theinfluence of temperature dependency of condenser 3 to low frequencies ofthe tuning range will only be comparatively very small. The influence oftemperature dependency of the additional condenser t is greater incontradistinction to that of condenser 3 as the capacity of theadjustable condenser I is higher, and will consequently be greater asthe adjusted natural frequency of the oscillatory circuit is lower.

Since the temperature dependency of condenser 3 strongly responds tohigh frequencies, but the temperature dependency of condenser 4 stronglyresponds to low frequencies of the tuning range it is possible to choosethe size and tem perature dependency of these condensers so that for oneof the higher frequencies and for one of the lower frequencies of thetuning range the natural frequency of the oscillatory circuit isindependent of the ambient temperature. As a result thereof theremaining temperature dependency of the oscillatory circuit is only verysmall throughout the tuning range.

The temperature dependency of the oscillatory circuit then availableappears from the graph represented in Fig. 2. The abscissa-axis hasplotted on it in kilocycles/second the natural frequency w of theoscillatory circuit (tuning range A), the variation no of the naturalfrequency occurring at a definite temperature variation of say 1 C.being plotted in cycles/second on the ordinate axis. The solid curve 0.indicates the natural frequency variation of the oscillatory circuitwhen using the circuit according to the invention. Therefrom it appearsthat in regard to the natural frequency the oscillatory circuit isindependent of the temperature for the frequencies in and wz. Forcomparison the natural frequency is indicated by a dotted curve b whenusing exclusively the condenser 3 connected in parallel with tuningcondenser I. As appears therefrom the use of the invention yields a verymaterial improvement in regard to temperature dependency of theoscillatory circuit.

When for tuning the oscillatory circuit a coil with adjustableinductance is used, two additional coils having a negative temperaturecoeflicient must be used in order to compensate for the naturaltemperature dependency of the oscillatory circuit according to theinvention. As is well known a negative temperature coefficient of a coilconsisting of several turns may, for instance, be obtained by suchmechanical attachment of the coil that due to an increase in temperatureits axial length becomes comparatively much greater. Similarly aniron-cored coil may be used with which the permeability of the coredecreases with increasing temperature. In the circuit shown in Fig. 3the oscillatory circuit consists of a fixed condenser 6 and anadjustable coil 5. A preferably small coil I having a negativetemperature coefficient is connected in series with the tuning coil 5.Furthermore, a preferably large coil 8 having a negative temperaturecoefficient is connected in parallel with the tuning coil. The coil 8,which is preferably larger than the maximum size of the tuning coil, hasan iron core whose permeability decreases with increasing temperature.

It is to be noted that coil I responds most strongly to highfrequencies, whereas coil 8 responds most strongly to low frequencies ofthe tuning range. Otherwise, the effect of the two additional coils Iand S is similar to that of both additional condensers 3 and 4 shown inFig. 1. The additional reactances of the tunable oscillatory circuit,which are used according to the invention, may be used for reducing thenatural temperature dependency of the oscillatory circuit, and, inaddition, for other purposes as will be set out, for instance, byreference to Fig. 4.

Fig. 4 represents a heterodyne receiving circuit of a well known type.The high frequency voltages taken from an antenna 9 are supplied to aninput oscillatory circuit III which can be tuned by means of anadjustable condenser Ill and is connected to the fourth control grid ofa mixing tube I I. The first control grid of the mixing tube is coupledwith an oscillatory circuit l3 by means of a grid condenser I2. Thisoscillatory circuit can be tuned by means of an adjustable condenser iswhich is adjusted in common with the tuning condenser Ill of the inputcircuit Ill. The coil i5 of the oscillatory circuit I3 is coupled insuch a manner with a coil I6 interposed in the circuit of the secondgrid of the mixing tube that local oscillations are produced.

As is well known it is often required for maintenance of a substantiallyconstant frequency difference between the signal input circuit and theoscillatory circuit of the local oscillator to connect a small condenserl9, which may be adjustable if desired, in parallel with the tuningcondenser I 4. Furthermore, it is necessary to displace the frequency ofthe input oscillatory circuit Ill and the oscillatory circuit I 3 by anamount corresponding to the desired intermediate frequency. Theintermediate frequency, to which the intermediate frequency bandpassfilter I8 interposed in the anode circuit of mixing tube I I is tuned,being invariable the frequency difference of the two oscillatorycircuits I0 and I3 must be kept as constant as possible. As is wellknown this constant frequency difference is secured by providing anadditional condenser IT in series with condenser I4 and coil I5 of theoscillatory circuit I3.

According to the invention the disturbing natural temperature dependencyof oscillatory circuit I3 is eliminated by giving condensers I7 and I9 anegative temperature coefficient. Since, as will be appreciated, in theexample shown in Fig. 4 the value of the capacity of both condensers IIand I9 generally cannot be chosen in accordance with the temperaturedependency, it is, of course, necessary for securing the desired effectin regard to elimination of the disturbing temperature dependency of theoscillatory circuit to choose the temperature dependency of each of thecondensers I1 and I9 accordingly. Sometimes it may be advantageous, forinstance if the size of condenser It can be determined only veryinexactly beforehand and consequently its influence on the temperaturedependency of the oscillatory circuit neither can be exactly determinedbeforehand, to connect in parallel with tuning condenser M a condenserhaving a fixed capacity but an adjustable temperature coefficient. Inthis case the oscillatory circuits I0 and I3 can be balanced by means ofcondenser I9, whereupon the temperature dependency of oscillatorycircuit I3 can be readjusted by means of the condenser with adjustabletemperature coefficient.

Fig. shows a tunable oscillatory circuit constituted by an adjustablecondenser l and a fixed 5 coil 2'. As already mentioned, the capacity ofthe condenser l and the inductance of the coil 2' generally increase atan increase in temperature. To reduce the resulting dependency ontemperature of the oscillatory circuit, the adjustit) able condenser l'according to the invention has an additional condenser 3 connected inseries with it, whose capacity decreases at an increase in temperatureor, to say it briefly, exhibits a negative temperature coefiicient. InFig. 5, the negative temperature coefficient of this condenser has beendenoted by a negative sign. For the condenser 3' use may, of course, bemade of different kinds of condensers with a negative temperaturecoefficient. To this end, use may be made, for example, ofthe condensersknown per se having a dielectric, whose dielectric constant decreases atan increase of temperature, or of condensers whose electrodes arearranged so as to be movable with respect to one another and, at anincrease in temperature, are mutually displaced by means of abi-metallic strip or the like in such manner that the capacitydecreases.

The capacity of the condenser 3 is preferably greater than the maximumcapacity of the tuning condenser. Particularly in this case it may beadvantageous with a view to saving cost to use the arrangement shown inFig. 6, in which the additional impedance used is constituted by theparallel connection of two condensers 4 and 5' of different size,instead of a single condenser with a negative temperature coefficient.The larger condenser 4 may in this case exhibit a slightly positivetemperature coefiicient, whereas the smaller condenser 5' exhibits sostrong a negative 40 temperature coefiicient that the total capacity ofthe parallel-connected condensers 4' and 5 exhibit the negativetemperature coefficient required to compensate the natural dependency ontemperature of the oscillatory circuit.

If for tuning the oscillatory circuit use is made of a coil, theinductance of which is adjustable the additional reactance used forcompensating the dependency on temperature must be preferably a verysmall coil 6' as is shown in Fig. 7. As 50 is well known, a negativetemperature coefficient of a coil constituted by a few turns may beobtained, for example, by such a mechanical construction of the coilthat an increase in temperature results in a comparatively greatincrease of 55 its axial length. Use may alternatively be made of a coilprovided with an iron core, in which the permeability of the coredecreases at an increase in temperature.

The additional reactance of the tunable oscilla- 60 tory circuit usedaccording to the invention may be used to reduce the dependency ontemperature of the oscillatory circuit and, in addition, for otherpurposes, as will be explained more fully, by way of example, byreference to Fig. 8. Fig. 8 shows a superheterodyne receivingarrangement of the kind known per se. The high frequency voltagesderived from an aerial 1 are supplied to an input oscillatory circuit 8'which can be tuned by means of an adjustable condenser 8" and isconnected to the fourth control grid of a mixing tube 9'. The firstcontrol grid of the mixing tube is connected to an oscillatory circuit Hby means of a grid condenser ID". This oscillatory circuit may be tunedby an adjustable condenser l2 which is 76 adjusted in common with thetuning condenser of the input circuit. A coil I3 of the oscillatorycircuit II is coupled to a coil 14' interposed in the circuit of thesecond grid of the mixing tube, in such manner that oscillations areproduced.

As is well known, it is necessary that the input oscillatory circuit 8'and the oscillatory circuit I I of the oscillator should be displaced infrequency by an amount corresponding to the desired intermediatefrequency. Since the intermediate frequency, to which the intermediatefrequency bandfilter l6 interposed in the anode circuit of the mixingtube 9' is tuned, is invariable, the difference in natural frequencybetween the two oscillatory circuits 8' and H must be maintained asconstant as possible. This can be achieved, as is well known, byproviding an additional condenser IS in series with the condenser l2 andthe coil l3 of the oscillatory circuit II.

According to the invention, a reduction of the natural dependency ontemperature of the oscillatory circuit I l' is now obtained by givingthe condenser l 5' a negative temperature coefficient. In view of thechoice of the additional reactance it should still be noted that acomplete compensation of the natural dependency on temperature of theoscillatory circuit due to the measure accord ing to the invention canonly be obtained for a single frequency of the tuning range, and thatthe detuning produced due to fluctuations of the ambient temperature isgenerally most disturbing for the higher frequencies of the tuningrange.

What is claimed is:

1. In a high frequency oscillatory circuit comprising a pair ofreactances of opposite sign, means for adjusting the magnitude of one ofsaid reactances thereby to adjust the frequency of the circuit over awide range of frequencies, a third reactance in shunt with saidadjustable reactance, a fourth reactance in series with both said pairof reactances, said third and fourth reactances being of the same signas said adjustable reactance, and said third and fourth reactances eachhaving temperature coefficients of the opposite sign to said adjustablereactance.

2. In a high frequency oscillatory circuit comprising a pair ofreactances of opposite sign, means for adjusting the magnitude of one ofsaid reactances thereby to adjust the frequency of the circuit over awide range of frequencies, a third reactance in shunt with saidadjustable reactance, a fourth reactance in series with both said pairof reactances, said third and fourth reactances being of the same signas said adjustable reactance, and said third and fourth reactances eachhaving temperature coemcients of the opposite sign to said adjustablereactance, and the magnitudes of said third and fourth reactances beingso related that the oscillatory circuit frequency is independent oftemperature for one of the higher frequencies and for one of the lowerfrequencies of said range.

3. In a high frequency oscillatory circuit comprising a pair ofreactances of opposite sign, means for adjusting the magnitude of one ofsaid reactances thereby to adjust the frequency of the circuit over awide range of frequencies, a third reactance in shunt with saidadjustable reactance, a fourth reactance in series with both said pairof reactances, said third and fourth reactances being of the same signas said adjustable reactance, and said third and fourth reactances eachhaving temperature coefficients of the opposite sign to said adjustablereactance, and said adjustable reactance being a condenser having apositive temperature coefficient.

4. In a high frequency oscillatory circuit comprising a pair ofreactances of opposite sign,

means for adjusting the magnitude of one of said rea'ctances thereby toadjust the frequency of the circuit over a wide range of frequencies, athird reactance in shunt with said adjustable reactance, a fourthreactance in series with both said pair of reactances, said third andfourth reactances being of the same sign as said adjustable reactance,and said third and fourth reactances each having temperaturecoefficients of the opposite sign to said adjustable reactance, saidadjustable reactance being a coil having a positive temperaturecoefficient.

5. In a high frequency oscillatory circuit comprising a pair ofreactances of opposite sign,

means for adjusting the magnitude of one of said reactances thereby toadjust the frequency of the circuit over a wide range of frequencies, athird reactance in shunt with said adjustable reactance, a fourthreactance in series with both said pair of reactances, said third andfourth reactances being of the same sign as said adjustable reactance,and said third and fourth reactances each having temperaturecoefficients of the opposite sign to said adjustable reactance, saidthird reactance having a magnitude larger than the maximum value of theadjustable reactance, and the fourth reactance having a value smallerthan the minimum value of the adjustable reactance.

DIEDERIK KOFFYBERG.

